The present invention relates to an elevator rope arrangement.
In traction sheave elevators, the elevator car and counterweight are suspended on round steel ropes. Normally, the same ropes act both as suspension ropes, whose function is to support the elevator car and counterweight, and as hoisting ropes serving to move the elevator car and counterweight. Therefore, the ropes must be designed to carry the entire load, even if, when a counterweight is used, the force needed to move the elevator is very smallxe2x80x94in an extreme case nearly zero when the counterweight and the elevator car with the car load are equal in weight.
In prior art, there are also solutions having separate suspension ropes and hoisting ropes. Such an elevator is presented e.g. in U.S. Pat. No. 5,398,781. In the elevator described in this specification, the suspension rope is attached to the top part of the elevator car and passed via diverting pulleys to a lever element on the counterweight. The hoisting rope is attached either to the top or bottom part of the elevator car and, like the suspension rope, passed via diverting pulleys and the traction sheave of the hoisting machine to a lever element on the counterweight. To compensate for rope elongation, the elevator described in this specification comprises a lever element fitted in conjunction with the counterweight and acting as a tensioning device. This patent focuses especially on the tensioning of the hoisting rope and contains no mention of any details of the suspension ropes or hoisting ropes. Neither does it describe any advantages that could be achieved by separate implementation of hoisting ropes and suspension ropes.
The hoisting ropes generally used are steel cables, whose friction coefficient is, however, so low that it has to be increased e.g. by using traction sheaves with different types of grooves or by increasing the angle of contact or angle of rotation of the rope around the traction sheave. In addition, a hoisting rope made of steel functions as a kind of sound bridge between the hoisting motor drive and the elevator car, transmitting noise from the hoisting machinery to the elevator car and thus impairing passenger comfort.
A further drawback with prior-art solutions using steel hoisting ropes is that the bending radius of the rope is relatively large, which means that the traction sheave and diverting pulleys must have a large diameter. Another drawback with steel rope is that the weight of the rope imposes a limit on the hoisting height of elevators. Moreover, steel ropes are liable to corrosion, so they require regular maintenance.
Specification EP 672 781 A1 presents a round elevator suspension rope made of synthetic fibers. Topmost on the outside it has a sheath layer surrounding the outermost strand layer. The sheath layer is made of plastic, e.g. polyurethane. The strands are formed from aramid fibers. Each strand is treated with am impregnating agent to protect the fibers. Placed between the outermost and the inner strand layers is an intermediate sheath to reduce friction. To achieve a nearly circular strand layer and to increase the volumetric efficiency, the gaps are filled with backfill strands. The function of the top-most sheath layer is to ensure a coefficient of friction of desired magnitude on the traction sheave and to protect the strands against mechanical and chemical damage and UV radiation. Thus, the load is supported exclusively by the strands. As compared with corresponding steel rope, a rope formed from aramid fibers has a substantially larger load bearing capacity and a specific weight equal to only a fifth or a sixth of the specific weight of corresponding steel rope.
A drawback with these prior-art solutions, in which a round elevator rope formed e.g. from synthetic fibers, is that the rope has a relatively large bending radius, requiring the use of large-diameter traction sheaves and diverting pulleys. Further, there occurs a fair deal of sliding of the strands and fibers in relation to each other. Moreover, the ratio of volume to area is high, which means that frictional heat will not be effectively removed from the rope and the rope temperature is therefore liable to rise unduly.
The object of the present invention is to eliminate the drawbacks of prior art and achieve a new type of elevator rope arrangement, in which the elevator ropes are divided into two categories: a) suspension ropes, whose function is to connect the elevator car and the counterweight to each other and to support them, and b) a new type of hoisting rope made of synthetic material, whose function is to receive the unbalance between the counterweight on the one hand and the elevator car and its load on the other hand and to move the elevator car.
In this arrangement, friction is not a necessary consideration regarding the suspension ropes, so these can be made of steel cable. contrast, the hoisting ropes are thin ropes of synthetic material, in which the tensile strength of the structure is formed by longitudinal strands of e.g. aramid fiber. These strands are surrounded by a sheath that binds the strands of each rope together and provides a good friction coefficient against the traction sheave. The sheath is made of e.g. polyurethane, which gives a multifold friction coefficient as compared e.g. with steel rope. Details of the features characteristic of the solution of the invention are given below.
The hoisting ropes now only have to bear a fraction of the loads of the elevator, as they need not support the load resulting from the passengers or goods being transported and the counterweight. Therefore, the elevator hoisting rope of the invention can be made very thin, which means that it has a small bending diameter. The hoisting rope can also be implemented as a flat rope, in which case the sheath of the hoisting rope is of a planar shape and, in cross-section, the hoisting rope thus has a width substantially larger than its thickness.
The thin and flat hoisting rope allows the use of a traction sheave that is considerably smaller in diameter and lighter than those used at present. Therefore, also the moment required for moving the elevator car is low, and consequently it is possible to use a small and cheap hoisting motor. The flat band-like shape of the rope distributes the pressure imposed by the rope on the traction sheave or diverting pulley more uniformly on the surface of the traction sheave. Further, sliding of the fibers relative to each other is minimised, and so the internal shear forces in the rope are also minimised. In addition, the ratio of volume to area is low, which means that frictional heat is effectively transmitted from the rope to the environment. Furthermore, the sheath of the hoisting rope can easily be coated with various materials, so the friction and abrasion characteristics can be optimised for different traction sheave materials. The small motor and small traction sheave are well applicable to an elevator without machine room because the hoisting motor with the traction sheave can be easily accommodated in the elevator shaft.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.